Certain materials are electroluminescent—that is, they emit light, and so glow, when an electric field is generated across them. The first known electroluminescent materials were inorganic particulate substances such as zinc sulphide, while more recently-found electroluminescent materials include a number of small-molecule organic emitters known as organic light emitting diodes (OLEDs) and some plastics—synthetic organic polymeric substances—known as light-emitting polymers (LEPs). Inorganic particulates, in a doped and encapsulated form, are still in use, particularly when mixed into a binder and applied to a substrate surface as a relatively thick layer; LEPs can be used both as particulate materials in a binder matrix or, with some advantages, on their own as a relatively thin continuous film.
This electroluminescent effect has been used in the construction of displays. In some types of these a large area of an electroluminescent (EL) material—generally referred to in this context as a phosphor—is provided to form a backlight which can be seen through a mask that defines whatever characters the display is to show. In other types there are instead individual small areas of EL material. Displays of either of these types have many applications; examples are a simple digital time and date display (to be used in a watch or clock), a mobile phone display, the control panel of a household device (such as a dishwasher or washing machine), and a handheld remote controller (for a television, video or DVD player, a digibox, stereo or music centre or similar entertainment device).
As noted above, the electroluminescent effect can be used to make a backlight that can shine through a mask defining a display. From front (the side from which it is to be viewed) to back such a backlight commonly consists of:                a relatively thick protective electrically-insulating transparent front layer known as the substrate and made usually of a glass or a plastic such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN);        over the entire rear face of the substrate, a relatively thin transparent electrically-conductive film made from a material such as indium tin oxide (ITO), this forming one electrode—the front electrode—of the backlight;        covering the rear face of the front electrode, a relatively thin layer of electroluminescent phosphor material (usually a particulate phosphor within a binder matrix);        over the rear face of the phosphor layer, a relatively thin electrically-insulating layer of a material—usually a ceramic—having a relatively high dielectric constant (relative permittivity) of around 50 (in some applications, such as the present invention, this layer is most desirably significantly optically-reflective, while in others it preferably has low reflectivity);        covering the entire rear face of the electrically-insulating layer, a continuous electrically-conductive film, usually opaque (and typically carbon or silver), forming the other electrode—the back electrode—of the backlight.        
In addition, the back electrode layer, which is quite delicate, is generally covered with a protective film (in some applications this is usually another, similar, ceramic layer, whereas in others—and in the present one—it is preferably a flexible polymeric material) to prevent the layer being damaged by contact with whatever device components—electronic circuitry, for example—might be mounted behind the display.
A number of techniques known to those skilled in the art may be used to construct such a device. However, each of the various layers is preferably screen-printed into place (apart from the ITO front electrode, which is usually sputtered onto the substrate), through masks that define the shape, size and position of the layer components, using suitable pastes that are subsequently dried, set or cured, commonly through the application of heat or ultraviolet light, as appropriate, prior to the next layer or collection of layers being applied. And in the context of electroluminescent displays, the expressions “relatively thick” and “relatively thin” mean thicknesses in the ranges, respectively, of 30 to 300 micrometers, usually around 100 micrometers, and 50 micrometers, and most usually 25 micrometers or less.
In a display, such a backlight is generally positioned behind a mask. Typically, such a mask is permanent—that is containing fixed, predefined, transmitting and blocking areas. The switching of such a display is controlled by turning the backlight, or sections thereof, on or off.